The present invention relates to door positioning systems for use in positioning doors, and particularly for use with doors that move with a rectilinear motion. More particularly, the invention relates to door positioning systems using pulleys, cables, and a lead screw to position rectilinearly moving doors.
Various applications such as sliding industrial doors in a manufacturing plant require that the doors be positioned rapidly. Conventional positioning systems for positioning doors include linear and rotary systems. Linear systems are available, but are typically high precision and, consequently, very expensive.
Rotary systems are a more economical alternative to linear systems. Rotary systems convert the rotary output of a motor to the desired linear door motion and are generally of the perpendicular or parallel type. In a perpendicular type system, the axis of rotation of the motor shaft is perpendicular to the direction of movement of the door. In a parallel type system, the motor shaft rotates about an axis that lies parallel to the line of motion of the door.
The basic form of the perpendicular motor system has a continuous loop of cable or chain running over a sheave mounted on the motor shaft (or on a shaft driven with the motor shaft). As the motor turns, it draws the cable or chain over the sheave, and the door moves with the cable (or chain). The many variations on this theme are associated with how the motor is integrated into the loop.
The method of placing the motor in the cable loop range from a single turn around the motor sheave, running through two idler pulleys, to a multi-turn windlass approach. One type of operator uses a single loop of cable with the motor at one end. This type of drive can only pull the door in one direction, because it is not possible to push the cable to drive the door in the other direction.
One of the disadvantages of a perpendicular cable drive, as usually practiced, is that the cable is not positively engaged with the driver sheave. Because of this, a larger contact pressure and total angle of contact are required to ensure that the cable does not slip on the sheave. Positive engagement is especially important if limit switches for the door travel are placed on the driver itself instead of on the door.
The basic forms of parallel motor systems are based on lead screws and threadless rods. The threadless rod principle involves a rod rotating within inclined rollers which are attached to the door. As the rod rotates, there is a component of the friction force which acts along the length of the rod. This component of the friction force causes the inclined rollers to travel back and forth along the rod, depending upon the direction of the rotation. As an example, a threadless rod concept is used by Horton Automatics on their Type 110 sliding door operator. A primary disadvantage of this type of drive system is the low available force. Only a component of the friction force is useful in driving the door, and it is therefore difficult to use this method for driving heavier doors at high speeds. For example, the Horton system provides a closing force of 30 pounds to the door. This would result in a very small acceleration on a 1200 pound door (approximately 10 in/sec.sup.2). With this acceleration, it would take approximately 50 seconds for the door to reach 48 in/sec, and over that time, the door would travel approximately 10 feet. The operating time for a practical range of door sizes would be too long.
In another parallel type system, the motor rotates a threaded member, or lead screw, to positively drive a nut towards or away from the motor. However, large doors would require long lead screws. Long unsupported lead screws have problems with critical speeds which limit the motor speed. Limited motor speed in turn limits the acceleration and speed of the door.
A positioning system that would provide adequate acceleration to move a door weighing 1200 pounds or more, yet having a configuration that occupies a small volume would be a substantial improvement over conventional positioning systems.
According to the present invention, a door positioning system for use with a generally rectilinearly movable door comprises a lead screw extending in the direction of a track system for the door, a motor for driving the lead screw, and a carriage for threadedly engaging the lead screw to move therealong when the lead screw is driven. The invention also includes means for amplifying the movement of the carriage so that the door moves a predetermined multiple of the movement of the carriage, wherein the amplifying means provides a driving connection between the carriage and the door.
According to one aspect of the invention, the lead screw includes external threads and the carriage includes a nut having internal threads for engaging the external threads. The nut moves the carriage axially along the lead screw in response to rotation of the lead screw. The amplifying means includes a plurality of pulleys coupled to the nut and a cable coupled to the pulleys.
According to another aspect of the invention, a housing substantially envelopes the positioning system and the amplifying means further includes a plurality of pulleys coupled to the housing. The cable couples the housing pulleys to the carriage pulleys and to the door so as to multiply the speed and distance of movement of the nut along the lead screw so that the door moves a predetermined multiple of the movement of the nut.
Other objects and advantages of the invention will become apparent from the following description of a preferred embodiment presently perceived as the best mode for practice of the invention and consideration of the accompanying drawings.